As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries as an energy source has also sharply increased.
Depending upon kinds of external devices in which such secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected to each other. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the output and capacity of one battery. On the other hand, a battery pack needs to be used in middle or large-sized devices, such as laptop computers, portable digital versatile disc (DVD) players, small-sized personal computers (PCs), electric vehicles and hybrid electric vehicles, because high output and large capacity are necessary for such middle or large-sized devices.
Such a battery pack is manufactured by connecting a protection circuit to a core pack having a plurality of unit cells connected in series and/or in parallel to each other. In a case in which prismatic or pouch-shaped batteries are used as the unit cells, the prismatic or pouch-shaped batteries are stacked so that large-sized surfaces of the prismatic or pouch-shaped batteries face each other, and then electrode terminals of the prismatic or pouch-shaped batteries are connected to each other by connection members. In a case in which a three-dimensional secondary battery pack having a hexahedral structure is to be manufactured, therefore, the prismatic or pouch-shaped batteries are preferably used as unit cells of the battery pack.
On the other hand, cylindrical batteries generally have larger electric capacities than the prismatic or pouch-shaped batteries. However, it is difficult to arrange the cylindrical batteries so that the cylindrical batteries have a stacked structure due to the external shape of the cylindrical batteries. In a case in which a battery pack is generally configured to have a line type structure or in a plane type structure, though, the cylindrical batteries are structurally more advantageous than the prismatic or pouch-shaped batteries.
Specifically, parallel-connection of cylindrical battery cells in a large-capacity battery pack is typically shown in FIG. 1, and thicknesses of a case of one of the cylindrical battery cells and a connection member are typically shown in a partial view of FIG. 2.
Referring to these drawings, cylindrical battery cells 10 are disposed so that electrode terminals 150 of the battery cells 10 having the same polarities are oriented in the same directions, and long bar type connection members 20 are connected to the electrode terminals 15 of the battery cells 10 having the same polarities by welding so that the battery cells 10 are connected in parallel with each other.
The case of each of the cylindrical battery cells 10 has a thickness t of approximately 0.2 mm. High current of 50 A or more flows in the connection members 20, by which the cylindrical battery cells 10 are connected in parallel to each other. For this reason, it is necessary for each of the connection members to have a thickness T of approximately 3 mm based on electrical or structural connection thereof.
In a case in which the connection members are connected to the cases of the respective cylindrical battery cells by welding, however, resistance difference occurs as the difference in thickness between the case of each of the cylindrical battery cells and each of the connection members increases with the result that it is difficult to connect the connection members to the cases of the respective cylindrical battery cells by resistance welding
Consequently, there is a high necessity for a high-output, large-capacity battery pack wherein contact resistance is minimized while manufacturing efficiency is improved.